The present invention relates generally to current sensing devices for electrical systems, and more particularly to underwater magnetic core assemblies for use therein.
Electrical power distribution systems may require the use of a variety of circuit condition monitoring devices to facilitate the detection and location of system malfunctions. Such devices include manually and automatically resetting current fault indicators, such as those manufactured by E. O. Schweitzer Manufacturing Co., and described in U.S. Pat. Nos. 4,288,743, 4,086,529 and 3,676,740 of the present inventor, as well as voltage monitoring devices, such as described in U.S. Pat. Nos. 4,251,770, 4,152,643, 4,263,350 and 4,550,288 of the present inventor. The devices are typically either of the test point mounted type for mounting on a system test point, or of the clamp-on type, for clamping directly onto a cable of the system. However, at times these devices may need to be somewhat remotely located from the cable of the system.
Clamp-on type fault indicators typically derive their operating power directly from current flow in the monitored conductor. In such current-reset fault indicators a magnetic core assembly is generally required to provide a concentrated magnetic flux from current flow through the conductor. A magnetic winding in magnetic association with the core assembly converts the concentrated flux to an electrical current which is rectified for use in powering the fault indicator.
For optimum flux pick-up it is desirable that the magnetic core assembly be in the form of a closed loop which completely encircles the conductor. However, to enable the core assembly to be installed and removed from the conductor, it is necessary that connection means be provided by which the loop can be opened. One form of magnetic core assembly which has proved particularly successful in this regard is described in U.S. Pat. No. 3,725,832, which issued to the present inventor on Apr. 3, 1973. In this construction a circular magnetic core is provided which comprises a plurality of laminations of pre-stressed oriented silicon steel secured together near their juxtaposed ends so as to form an annular loop around a monitored conductor. The ends are provided with interlocking tongue-and-groove type members formed of corrosion resistant magnetic material which allow the loop to be opened for installation and removal on a conductor.
One requirement of closed loop magnetic core current-reset fault indicators having magnetic sensing means such as a reed switch or magnetic sensing coil for fault detection is that adequate coupling be maintained between the magnetic sensing means and the monitored conductor. Because there are only a limited number of different sized cables that are left underwater, the present invention is designed to emcompass one of these dimensions whereby the monitored cable is always maintained in close proximity to the magnetic sensing means of the fault indicator.
Similarly, closed loop magnetic core assemblies have been susceptible to being inadvertently pulled off the monitored conductor, as when force is exerted on leads connected to the circuit module. But again, because the present invention is designed for a specific size of underwater electrical cable and in effect produces a locked engagement, removal is possible only by deliberate manipulation of the core assembly.
The most significant drawbacks of prior art closed loop magnetic core assemblies for use with underwater electrical cable is their corrosive and/or collection of sedimentary deposits properties. On the one hand if the assembly were to corrode and subsequently breakdown via an underwater chemical reaction, then the cable could not be maintained in close proximity to the magnetic sensing means. Conversely, if sediments were to deposit on the assembly it could effect current flow as well as prevent the assembly from being removed from its original attachment and placed downline or on another cable altogether. The present invention obviates these problem's by providing a construction whereby the magnetic core assembly is fully submerseable for an indefinite amount of time while immune from collecting harmful deposits and/or corroding.
Accordingly, it is a general object of the present invention to provide a new and improved underwater current transformer.
It is a more specific object of the present invention to provide a new and improved clamp-on type underwater current transformer having an improved magnetic core assembly.
It is another object of the present invention to provide an underwater current transformer which is less susceptible to being inadvertently pulled free of the monitored cable.
It is another object of the present invention to provide an underwater current transformer which is fully submerseable for an indefinite amount of time while immune from collecting harmful deposits and/or corroding.
It is another general object of the present inventin to provide a new and improved underwater current transformer for use in fault indicators and the like.